KEY LARGO, Fla. – Last year, in the third week in August, a band of researchers armed with nets and buckets jumped into the warm water to capture millions of baby coral.

PAIGE GILL / Florida Keys National Marine Sanctuary

Rising ocean temperatures, pollution and other factors may eventually render many coral species, like this elkhorn off Florida, extinct.

By the dim light of a crescent moon, they dived into the shallow, warm water and watched as millions of milky white coral spawn burst from the spiny creatures. Their goal was to study how coral reproduce. It's a race against time: Coral reefs are dying at an unprecedented rate. As many as one-fifth of the world's coral reefs have already been destroyed, according to a 2004 study, and the health of coral in Florida is declining.

So in August 2004, the researchers took the coral spawn back to the lab, watched each grow to the size of a grain of Cream of Wheat, then replanted thousands of the spawn back on the reef.

Fast forward one year.

Last month, the same researchers visited the same reef in Florida Keys National Marine Sanctuary off Key Largo. They were horrified at what they saw: Last year's four hurricanes, hungry fish and high water temperatures had ravaged the reef. All the spawn were dead.

Except one.

They named him Harold.

He's an elkhorn coral, about the size of a fingertip. Right now he's kind of yellowish, but his color will ripen into a vibrant hue if he makes it to adulthood. He's also the first elkhorn coral born in the wild, raised by researchers, then returned to the reef.

Harold's existence is more than just an academic experiment. Conditions are worsening for Florida's coral reefs, experts say. Coral aren't reproducing fast enough to replace what's dying. If researchers can find a way to help coral reproduce, then maybe they can save the reef.

"If we can enhance that survival even by a fraction, it's an improvement," said Brian Keller, science coordinator of Florida Keys National Marine Sanctuary.

Reluctant spawners

Coral has always been something of a mystery to marine biologists. In fact, no one knew exactly how – or when – coral reproduced until 1982.

Alina Szmant, a marine biologist at the University of North Carolina at Wilmington, was one of the researchers who discovered that coral reproduced on only one or two nights of the year.

"It took a while to figure out," said Szmant, who studied reefs in Puerto Rico this year. "We eventually narrowed down the time of year the corals would actually spawn."

Researchers like Szmant also learned that the world's coral don't reproduce at the same time. The Great Barrier Reef in Australia, for instance, reproduces in October. The coral in the Keys reproduces before the coral in Belize.

Of the millions of coral that spawn, only a few survive even in the best of conditions.

Coral are notoriously slow-growing. An elkhorn coral like Harold, for instance, grows a couple of inches a year – and that's a fast-growing species.

In centuries past, if some coral didn't reproduce, or if only a few lived to adulthood, it was probably no big deal. The strong genes survived.

These days, the biggest threat to the reefs is bleaching. That's the word scientists use when the coral lose their color and become starkly white. (Coral get their color from tiny algae that live inside the skeleton.)

Bleaching – triggered by high water temperatures, pollution or diseases – is becoming more common. When the coral are stressed, the algae inside die or are expelled. Experts say the coral reefs in the Keys are on the cusp of a "bleaching event."

About 30 percent of coral off the Keys died after one mass bleaching in the 1990s. Keller and others fear that may happen again this year; they have noticed that the fire coral is white, and that species is something of a proverbial canary in a coal mine for the rest of the reef.

"We've seen widespread paling in just about every type of coral out here," said Keller. "The entire Florida reef tract is at risk."

Synchronized spawning

Back at the lab in August, Margaret Miller peered into a microscope at some magnified pink blobs.

"These do look good," she said. "This is worthwhile."

She was looking at coral spawn that she hoped would become Harold's reefmates.

She barely hid her disappointment. The night before, they had gone out to collect but had gathered precious little spawn.

It was the third week in August, and by this time, she had hoped to have several coolers full of coral spawn, not just a few tablespoons in one Tupperware cup.

Miller, a 36-year-old ecologist at the National Oceanic and Atmospheric Administration and the University of Miami, had prepared all year for this research trip in the Keys. She and a dozen other scientists rented a two-story vacation house, amassed a mountain of dive gear, fired up several computers and unpacked who knows how many test tubes.

The newly collected spawn were to be placed in a large cooler, the kind that people take on a picnic. At the bottom of the cooler were some reef rocks. Miller hoped that the spawn would do what they normally would in the wild: float downward and attach themselves to the rocks.

They needed far more spawn if they had any hope for success. Miller wanted to go back into the water to gather more in hopes of improving their odds.

But prospects for the next few days were dim: Hurricane Katrina was bearing down.

Even during the best of times, fertilization and attachment to the reef are precarious events.

An egg from Coral A can't fertilize with the sperm from Coral A. The eggs and sperm from Coral A must be fertilized by the eggs and sperm from Coral B.

So if Coral A spawns on one night, and Coral B spawns the next, there is no fertilization. And even if the coral spawn successfully, they may get eaten by fish or not make it to a stable, rocky bottom.

Last year, there was plenty of synchronized spawning, Miller said.

This year, not as much.

"We were at a big thicket of coral, but they were all the same genetic individuals," she said. "There's just a lot of circumstances beyond our control."

Now that so many of the world's coral reefs are in peril, each year the coral don't spawn successfully is a catastrophe, Miller said.

Szmant and Miller are among a worldwide network of researchers studying how biologists can help save the reefs.

"(What we are doing) is a Band-Aid," Szmant said. "We're putting our finger in the hole in the dike."

Others, including the Nature Conservancy and the state of Florida, are spearheading a new program to measure the health of the entire Florida reef and identify those areas most resilient to bleaching. They will focus on protecting those areas most likely to survive, and try to replicate the factors that make them more resilient.

"What we're trying to do is sort of overcome those issues for the coral, kind of a captive breeding program," said Szmant. "We want to increase survivorship of the offspring, trying to counteract a low chance of fertilization."

After Hurricane Katrina blew through Florida, Szmant flew in from Puerto Rico, carrying some spawn of a different species from a reef down there.

Miller had some bad news for Szmant: They had only collected a measly 2,000 elkhorn coral spawn from the Keys reef.

So the team decided to switch gears and introduce some of the Puerto Rican coral spawn on a reef in the marine sanctuary.

On Friday, the hurricane well past, the team went to check on the spawn from Puerto Rico.

They also wanted to visit Harold, to see how he fared during the storm.

"Harold is doing great," Miller wrote in an e-mail. "And the cool thing is, at that time, I found a second survivor from last year.